Mobile wireless systems rely on efficient handoff algorithms in order to enable subscriber mobility while guaranteeing uninterrupted connectivity and required quality of service (QoS). Based on the frequency channels involved in the handoff process, handoff algorithms can be broadly classified as inter-frequency handoffs and same-frequency handoffs. Inter-frequency handoffs result in the mobile station resuming the current communication session on a different frequency at the end of the handoff process. Same-frequency handoffs result in the mobile station resuming the current communication session over the same frequency subsequent to the completion of the handoff process. In other words, in inter-frequency handoffs, the mobile station re-establishes connection via a different infrastructure node that operates on a frequency different from the one used by the mobile station before the handoff. In same-frequency handoffs, the mobile station re-establishes connection via a different infrastructure node that operates on a frequency exactly the same as the one used by the mobile station before the handoff.
Handoff algorithms are in widespread use in cellular systems. Early second generation (2 G) systems such as a Global System for Mobile Communication (GSM), implement inter-frequency handoffs in which different base stations transmit different frequencies on the downlink, and the mobile station measures the received signal quality of the active base station and up to six other neighboring base stations and reports the measurements to the active base station periodically. The mobile station performs these measurements by tuning its frequency to that of neighboring base stations during predefined time intervals when it does not exchange user data. The active base station forwards the measurements to a higher network entity, such as the base station controller. The base station controller determines from the measurements whether a handoff is necessary. If a handoff is deemed necessary, the base station controller selects the most suited base station from the list of measured base stations. The base station controller then informs the mobile station about the selected base station and its transmission parameters through the active base station, and directs the mobile station to handoff to the selected base station.
Cellular Digital Packet Data (CDPD) systems also perform inter-frequency handoffs by initiating channel scanning when the active channel is perceived to be unacceptable. Standards based on the code division multiple access (CDMA) technology, such as Interim Standard (IS)-95 and the more recent CDMA 2000-based and Universal Mobile Telecommunications Systems (UMTS)-based third generation (3 G) variants perform same-frequency handoffs. Both the International Institute of Electrical and Electronics Engineers (IEEE) Standard 802.11 and the High Performance Radio Local Area Network (HIPERLAN) 2 Standard support inter-frequency handoffs. They require the mobile stations to interrupt active communication sessions and scan alternate channels for possible beacon frames from other access points.
There has recently been considerable interest in the field of inter-system handoff and vertical handoff algorithms. Many fourth generation (4 G) networks propose seamless handoff between dissimilar systems like cellular and wireless local area networks (WLAN). In such systems the mobile stations are required to have the capability to transmit and receive the signals of the multiple systems involved. In most cases, mobile stations are required to have separate receivers for each communication system.
Some communication systems use a separate dedicated channel on which all base stations advertise their presence. A mobile station that requires performing a handoff need not scan the whole frequency band to identify suitable handoff candidates. Instead a mobile station only scans this single channel used for advertisements, to scan for suitable handoff candidates. Although this process is less time consuming compared to scanning multiple channels, the process does tend to waste bandwidth resources, especially if handoffs are not very frequent.
All forms of inter-frequency handoff algorithms discussed above require the mobile station to interrupt the active communication session to scan one or more other frequencies to listen for other infrastructure nodes that might be targeted for handoff. Therefore, such systems waste significant amounts of transmission time scanning for handoff candidates when actual user data could be transmitted. Such systems also risk losing significant amounts of information on the active communication channel, while their receiver is busy scanning other frequencies to assess potential handoff candidates. In order to avoid having to interrupt the active communication session, certain handoff algorithms require the use of one or more secondary receivers to perform simultaneous measurements on frequencies other than the active frequency. Such systems utilize the extra receiver or receivers to scan all channels or the dedicated handoff channel continuously, which eliminates the need for the primary receiver to go off-channel for scanning and thereby eliminates the risk of data loss. However, equipping mobile nodes with multiple receivers increases their cost as well as complexity.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.